The present invention relates to self-testing of microelectromechanical (MEMS) sensor devices and especially to a discrete-time high voltage generating circuitry as defined in the independent claim 1. The present invention further relates to a method to operate a discrete-time high voltage generating circuitry as defined in the independent claim 13.
Micro-Electro-Mechanical Systems or MEMS can be defined as micro-scale mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. MEMS structures can be applied to quickly and accurately detect very small changes in physical properties.
Capacitive microelectromechanical sensors have become part of many consumer devices and they are used also in a variety of safety critical applications, such as electronic stability control (ESC) in vehicles. Especially in the safety related applications, it is important to identify potential failures in mechanical or electrical signal paths of the capacitive sensor.
A capacitive sensor comprises at least one microelectromechanical element that comprises at least one capacitive element. The capacitive element comprises a rotor mass (a.k.a. proof mass or in short, just a rotor) and a stator which remains stationary while the rotor mass moves in response to acceleration. The position of the rotor mass in a reference system is measured by detecting signal capacitance. An electrode attached to or incorporated by the rotor mass and an electrode attached to or incorporated by the stator form a variable capacitor with a capacitance. When the rotor mass moves relative to the stator, a change in the distance between the electrodes is converted to a change in the capacitance of this variable capacitor. A single variable capacitor is created between a static electrode of the stator and a moving electrode of the rotor mass (rotor). The total capacitance of the single variable capacitor includes a static capacitance defined by the capacitor configuration and a signal capacitance that results from the motion of the rotor mass in response to external acceleration.
Built-in functionality diagnostics is a way to ensure that a device may identify its own erroneous operation rapidly. Recognizing erroneous operation or failure of the device is especially important for devices which are used for critical functionalities. An example of devices with such critical functionality is accelerometers in automotive components. A start-up self-test is run prior to normal operation and can be used to check at least the whole dynamic operating range of the sensor or even above the dynamic operating range of the sensor. The start-up self-test is thus capable of detecting for example faults which appear only above signal levels that are normally utilized but still within operating range.